Disease and pathogenesis

The clinical signs and symptoms are a consequence of the extent and frequency of infection, larval migration, and the host response. VLM or visceral toxocarosis seems to be associated with the host immune response to a large number of larvae migrating in the tissue. It is characterized by high eosinophilia, usually more than 30%, fever, hepatosplenomegaly, lymphadenopathy, hypergammaglobulinemia, respiratory signs, abdominal pain, and elevated titres of blood group isohaemagglutinins. Neurological manifestations have also been observed, such as convulsions, and larvae have been found in brains of patients at postmortem (Hill et al. 1985; Nelson et al. 1990). However, the results in a case-control study by Magnaval et al. (1997) suggest that migration of the larvae in human brain does not frequently induce a recognizable neurological syndrome.

OLM or ocular toxocarosis is believed to be caused by a lower infective dose, that is, fewer larvae migrate in the tissue. Persistent eosinophilia, leukocytosis, and elevated isohaemagglutinin levels commonly associated with VLM usually do not occur with ocular disease. Many patients may not have any clinical events except when a larva enters the eye. The patients are usually also older compared to patients presenting VLM (Ljungstrom and van Knapen 1989; Gillespie et al. 1993). Visual loss is usually unilateral and the degree is variable from dimness of vision through to blindness. Initially, granulomas, due to the inflammatory response, were the most frequently reported complication, but with improved diagnosis lesions other than the classical posterior pole granuloma have been reported (Shields 1984). OLM is an important differential diagnosis of retinoblastoma in children (Shields et al. 1991). Patients with covert toxocarosis (CT) may or may not present eosinophilia and when present it is generally not very high (Taylor et al. 1987, 1988). This syndrome is associated with positive serology and the clinical signs are common and non-specific but together they form a recognisable symptom complex. Recurrent abdominal and limb pain, lack of energy or tiredness are often recognized in covert toxocarosis but the same symptoms are also observed in patients suffering VLM. In asymptomatic toxocarosis, positive serology with or without eosinophilia can occur in the absence of other symptoms or signs. That asymptomatic toxocarosis is common is indicated by the relatively high seroprevalence of the general population in all countries of the world, although to some extent cross-reactions cannot be ruled out.